2. • Objectives
• 1. Relate the importance and methods
of counting populations.
• 2. Discuss why populations change in
size.
• 3. Explain how limiting factors affect a
population.
3. Population Size and
Arrangement
• A change in size of one population will
cause a change in size of another
population
• For this reason scientists want to know
of any increases or decreases of any
population they study.
• Finding the size is not always easy
4. • Animals can be marked with ear tags, or
transmitters and birds with leg bands.
• Trees are counted by marking them with paint
or ribbons.
• Populations are not spread out evenly but are
clumped.
• Bring clumped is beneficial because species
can help each other.
• Animals live in groups for protection. Plants
are protected from high winds by growing
together.
5. Population Changes
Population can change for several
reasons.
1. If the number of births goes up and the
number of deaths go down, then the
population will increase
2. The population will decrease if there
are more deaths than births.
6. 3. Animals can move from place to place.
Moving of animals out of an area is
called emigration. This would cause a
decrease in population.
4. When animals move out of one area
and into another, it is called
immigration. This causes an increase
in population.
7. Limits of Population Size
• Any condition that keeps the size of a
population from increasing is called a
limiting factor.
• Lack of light, space, water, or food can
be limiting factors.
8. • As the population increases and shelter, food
or water is used up, there becomes a struggle
for survival among the animals. This is called
competition.
• Because of the lack of these items,
population growth slows down by decreasing
the number of births and increasing the
number of deaths.
• Plants that grow close together compete for
minerals, water, space and sunlight.
• A decrease in these factors causes plant
growth to slow. This is why gardeners pull
weeds or thin out new seedlings.
